Doctors have identified a new ligament in the knee joint. While this structure had been previously described (as long ago as the 1800s), it is only now that the ligament has been clearly defined. Does it matter?
The doctors who describe the ligament think it does, and they have a good point. When people injure their knee ligaments, the injuries are much more complex than we often appreciate. Having a torn ACL often means much more than just injury to one ligament. For example, we know people who tear their ACL often damage cartilage, injure the meniscus, tear other ligaments, sustain bone bruises, and other injuries. Most surgeons agree that the diagnosis of an isolated ACL tear is a dramatic simplification of a very complex injury. People have an easier time understanding when a single ligament is discussed, but this is a much more complicated problem when examined closely.
One reason some people don't fully recover after ACL injury, is that this other damage in the knee was not recognized and treated. For example, posterolateral rotatory instability of the knee joint is one common reason that people don't fully recover from ACL surgery. It could be the case that damage to this "new" knee ligament may be another reason for persistent knee instability after ACL surgery.
The recently identified ligament is called the anterolateral ligament, or ALL, and is closely related to the lateral collateral ligament. The doctors are theorizing the ALL contributes to rotational stability of the knee, and may be one of the primary restraints to the pivot shift test, a common test for stability of the knee joint.
Sources: Paddock C. "Surgeons describe new knee ligament" Medical News Today. 8 November 2013.